The invention relates to antennas for use in portable communications devices such as mobile telephones. Portable communications devices are required to be compact in size, which is a requirement that applies to every component of the devices, including the antenna. Modern mobile telephones use two or more distinct frequency bands, and preferably one and the same antenna is required to operate in all frequency bands used by the telephone.
Currently, most mobile telephones use one or more of the following three frequency bands: the GSM band centred on the frequency 900 MHz, the DSC band centred on 1800 MHz, and the PCS band centred on 1900 MHz. The 900 MHz and 1800 MHz frequency bands are separated by one octave, whereas the 1800 MHz and 1900 MHz frequency bands are separated by only a fraction of one octave. In many mobile telephones using the 900 MHz and 1800 MHz frequency bands the antenna has separate portions tuned to respective ones of the two frequency bands, since it is not considered feasible to have one and the same portion of the antenna tuned to a frequency band of more than one octave with a relatively large unused frequency band between the useful frequency bands.
On the other hand, the two U.S. patent applications Ser. Nos. 09/112152 and 09/212259 describe attempts having been made to have one and the same portion of the antenna cover both high frequency bands centred on 1800 MHz and 1900 MHz with a lower frequency limit of 1710 MHz and an upper frequency limit of 1990 MHzxe2x80x94a bandwidth of 280 MHz. The improvement in bandwidth is obtained at the expense of antenna gain.
It is the object of the invention to provide an antenna, which is usable in at least three frequency bands and which has a minimum loss, ie maximum gain in all frequency bands.
The invention provides an antenna for use in eg portable communications devices such as mobile telephones. The antenna is useful in a low frequency band and two high frequency bands, where the two high frequency bands are relatively closer to each other than to the low frequency band. The antenna is thus effectively a triple band antenna, and a mobile telephone having such an antenna is thus useful in three frequency bands such as the above identified three frequency bands centred on 900 MHz, 1800 MHz and 1900 MHz respectively. However, the invention is not restricted to the use in the above-identified frequency bands, but will be suitable for use in existing and future frequency bands as well.